CD43 is an abundant heavily glycosylated protein expressed on the surface of leukocytes and platelets. The molecule appears critical to the processes of signal transduction and adhesion. Defective pathologic expression of CD43 has been linked to the development of Immunodeficiency diseases and physiological expression appears linked to inflammation and tissue injury. Consistent with its role in immune function, expression of CD43 is regulated by cytokines, antibodies and phorbol esters which induce cellular activation. The work proposed in this application is based on the hypothesis that regulation of CD43 gene activity is critical to the control of CD43 expression and, therefore, to immune function. Already, a 10l bp region of the CD43 gene has been defined which confers tissue- specific expression in tumor cell lines representing megakaryocytes, myelomonocytes and pre-erythrocytes. This promoter region also confers expression which is up-regulated during myeloid and megakaryocytic differentiation. The CD43 gene promoter is novel among non- housekeeping genes in containing no "TATA" or "CAAT" elements. To date three nuclear proteins which interact with the 10l bp promoter have been identified, all of which appear novel. Two factors, MS-2 and MS-A, are developmentally regulated in myeloid cells and one, MS-B, appears myeloid specific, MS-2 interacts with a wide range of gene promoters and analysis of candidate clones indicates that it may represent the founding member of a new class of transcription factors. The Aim of the current proposal is to extend these preliminary studies in order to determine the transcriptional mechanisms which regulate CD43 gene activity in normal non-transformed cells. First, alterations in gene activity in response to cellular activation will be analyzed. Second, the cis-acting elements and trans-acting factors responsible for this activity will be identified and characterized. Next, those trans- acting factors identified as novel will be cloned and assays performed to determine how they interact with each other and with components of the basal transcription machinery to elicit cell specific and developmental expression. Finally, those genes which encode factors that are regulated such as to suggest an involvement in hematopoietic differentiation will be "knocked- out". The effect of these "knock-outs" on hematopoiesis will be assessed in a newly described embryonic stem cell differentiation system. The effect of experimentally engineered induction of these genes will also be assessed. The studies proposed will define the mechanisms underlying transcriptional regulation of CD43. The knowledge gained will be invaluable in the development of novel therapeutic reagents which can modulate expression of CD43 in pathologic states such as Immunodeficiency and in physiologic suites such as ischemia which can lead to life threatening tissue damage.